A bone density test measures how strong and solid your bones are, primarily to diagnose osteoporosis, assess your risk of fractures, and track whether treatment is working. The test is quick, painless, and uses an extremely low dose of radiation to scan key areas like your spine, hip, and sometimes your forearm.
What the Test Actually Measures
The standard bone density test uses a technology called dual-energy X-ray absorptiometry, commonly known as a DXA or DEXA scan. The machine sends two low-energy X-ray beams through your bones and measures how much energy passes through versus how much is absorbed. Denser, stronger bone absorbs more energy. The result is a precise measurement of your bone mineral density (BMD) at the sites most vulnerable to fractures.
Most scans focus on the lumbar spine and hip, since fractures in these areas carry the most serious health consequences. A hip fracture in an older adult, for instance, can lead to long-term disability or loss of independence. Some scans also measure the forearm, and newer DXA systems can assess vertebral fractures, body composition, and even calcification in the abdominal aorta.
Who Should Get Tested
The U.S. Preventive Services Task Force recommends routine screening for all women aged 65 and older. Postmenopausal women younger than 65 should also be screened if they have risk factors such as low body weight, a parent who fractured a hip, smoking, or heavy alcohol use. For men, there is currently no universal screening recommendation because the evidence on benefits and harms is still inconclusive, though doctors may order a test based on individual risk.
These guidelines apply to adults 40 and older who have no known osteoporosis or history of fractures from minor falls or impacts. If you have an underlying condition like cancer, metabolic bone disease, or hyperthyroidism, or if you take medications associated with bone loss, different rules apply. Patients starting glucocorticoids (a type of steroid used for inflammation) at any dose for three months or longer should have their bone density measured at baseline, regardless of age or sex.
How Results Are Scored
Your results come back as a number called a T-score, which compares your bone density to that of a healthy young adult at peak bone mass. The scale works like this:
- T-score of -1 or higher: Normal, healthy bone density.
- T-score between -1 and -2.5: Osteopenia, meaning bone density is lower than normal but not yet in the osteoporosis range.
- T-score of -2.5 or lower: Osteoporosis, indicating significantly weakened bones and higher fracture risk.
The numbers are negative because they represent how far below peak bone density you fall. A score of -1.5, for example, means your bones are less dense than the young-adult average but still above the threshold for osteoporosis. The further below -2.5 your score drops, the greater the concern.
Your doctor may also use your T-score alongside a fracture risk calculator called FRAX, which factors in your age, weight, smoking status, family history, and other variables to estimate your percentage chance of breaking a bone in the next 10 years. This combined picture often guides decisions about whether to start medication or simply monitor your bones over time.
Tracking Treatment Over Time
If you’re diagnosed with osteoporosis and begin treatment, repeat bone density scans help your doctor see whether your bones are responding. The size of the improvement in bone density correlates closely with how much your fracture risk decreases, so these follow-up scans provide real, measurable feedback. Most people on treatment have a repeat scan every one to two years.
Timing also matters if treatment is paused. After stopping certain medications, bone density can start to decline again. How quickly that happens depends on the specific drug. Some medications hold their effect for two to three years after stopping, while others lose their protective benefit within a year. Your doctor will schedule repeat scans based on which treatment you were taking.
What to Expect During the Scan
A DXA scan is one of the simplest medical tests you can have. You lie fully clothed on a padded table while a scanning arm passes over you. There are no injections, no enclosed spaces, and no discomfort. The whole process typically takes 10 to 20 minutes.
The radiation exposure is remarkably low. A single DXA scan delivers about 0.001 millisieverts, which is roughly one-hundredth the dose of a standard chest X-ray. For context, you absorb more radiation during a few hours of normal daily life from natural background sources than you do from a bone density scan.
One preparation step to keep in mind: stop taking calcium supplements or multivitamins containing calcium for 24 hours before your appointment. Calcium in your digestive tract can interfere with the accuracy of a spine measurement. You should also avoid wearing clothing with metal zippers, buttons, or belt buckles over the areas being scanned, though most facilities will simply ask you to remove those items.
Why Early Detection Matters
Osteoporosis develops silently. You lose bone density gradually over years without any symptoms until a fracture happens, often from something as minor as bending over or coughing. By the time a fracture occurs, significant bone loss has already taken place. A bone density test catches the problem years before that first break, when lifestyle changes and treatment can make the biggest difference.
Even a result in the osteopenia range is useful information. It tells you your bones are trending in the wrong direction, giving you a window to increase weight-bearing exercise, improve your calcium and vitamin D intake, address modifiable risk factors like smoking, and establish a baseline for future comparison. Knowing where you stand turns a preventable crisis into a manageable number on a chart.